The present invention relates to voltage follower circuits.
The conventional voltage follower circuit is comprised of a pair of transistors having opposite polarities to each other and two electric current sources. The conventional voltage follower circuit will be explained in detail with reference to the drawings. FIG. 1 is a circuit diagram showing one example of the conventional voltage follower circuit.
Referring to FIG. 1, the voltage follower circuit includes NPN transistor Q11 having a collector connected to a positive power source terminal 1, a base connected to an input terminal 3, and an emitter connected to an electric current source I.sub.1. PNP transistor Q12 is connected at its base to the emitter of transistor Q11, connected at its collector to a negative power source terminal 2, and connected at its emitter to an electric current source I.sub.2 and to an output terminal 4.
When an input voltage is indicated by V.sub.IN and an output voltage is indicated by V.sub.OUT, the output voltage V.sub.OUT is represented by the following relations: ##EQU1## EQU .thrfore.V.sub.OUT .apprxeq.V.sub.IN ( 2)
where
V.sub.T : thermal electromotive voltage (.apprxeq.26 mV at t=25.degree. C.) PA1 Is(Q11): reverse saturation current of transistor Q11 PA1 Is(Q12): reverse saturation current of transistor Q12
In the relation (1), the second and third terms determine an offset voltage; however, as shown by the relation (2), practically the output voltage V.sub.OUT is approximately equalized to the input voltage V.sub.IN so that the conventional circuit performs the voltage follower function.
In the above described conventional voltage follower circuit, the values of Is(Q11) and Is(Q12) are not essentially matched to each other since the transistors Q11 and Q12 are of different types of NPN and PNP. Further, since the transistors Q11 and Q12 are not perfectly matched to each other with respect to their temperature characteristics, the offset voltage of output voltage V.sub.OUT is amounted up to several 100 mV relative to the input voltage V.sub.IN, thereby causing a drawback that the conventional circuit does not have good temperature characteristic. Moreover, there is another drawback such as the offset voltage fluctuates due to change of a base-emitter forward voltage V.sub.BE of the transistor Q12 depending on an output load current.